To make a breakthrough to the Zenith Heaven stage, one has to first open 360 immortal apertures (open 252 immortal apertures in addition to the already opened 108 immortal apertures) and then need the extremely precious Profoud True Essence to transform their Golden Immortal soul into a Zenith Heaven soul.
The Profound True Essence was extremely precious. It was said that one would only be able to find a sliver of it in the most extreme and pure places of the heavens and earth after passing through the astral winds of the nine heavens and entering the extremely dangerous outer space.
Advancing from the Taiyi stage to the Zenith Heaven stage was not only a breakthrough in cultivation, but more importantly, one's soul would undergo a massive transformation and begin to fuse with the physical body. Enceforth, every single movement of a Zenith Heaven cultivator would possess an overwhelming power of the soul.
To cultivate even further at the Zenith Heaven realm one has always have to open up immortal apertures but they have to also slay Corpses, a total of three impurities in a Zenith Heaven Immortal that shall be cut from him (from his soul) and suppressed to achieve perfection in his own being and comprehension of the law of heaven and earth, there a total of three of them for each bottleneck, one to reach middle stage (840 apertures), upper stage (1320 apertures) and peak stage (1800 apertures), making it so that it is know as slaying the Three Corpse (Evil, Good and Self Corpse)
Corpse slaying can be divided into two parts. The first step would be to sense the presence of the three corpses in my body. The second step would be to realize one's true nature and comprehend the will of the three corpses before slaying them..
When one cut a Corpse it produce another self representing the impurity that was cut and possessing the same cultivation as one self, but without the physical cultivator's body. This other self would always have the desire to replace him to take the original body that would boost their own cultivation. It is so that when one fails to suppress one of the corpses, there is an equivalent of a Zenith Heaven Immortal wanting to replace him at any moment (in total then three Zenith Heaven Immortals); suppressing the Three Corpse is the only way to get rid of them, as killing them would be the same as harming oneself.